Biogeography of Parasitic Nematode Communities in the Galápagos Giant Tortoise: Implications for Conservation Management

Abstract

The Galápagos giant tortoise is an icon of the unique, endemic biodiversity of Galápagos, but little is known of its parasitic fauna. We assessed the diversity of parasitic nematode communities and their spatial distributions within four wild tortoise populations comprising three species across three Galápagos islands, and consider their implication for Galápagos tortoise conservation programmes. Coprological examinations revealed nematode eggs to be common, with more than 80% of tortoises infected within each wild population. Faecal samples from tortoises within captive breeding centres on Santa Cruz, Isabela and San Cristobal islands also were examined. Five different nematode egg types were identified: oxyuroid, ascarid, trichurid and two types of strongyle. Sequencing of the 18S small-subunit ribosomal RNA gene from adult nematodes passed with faeces identified novel sequences indicative of rhabditid and ascaridid species. In the wild, the composition of nematode communities varied according to tortoise species, which co-varied with island, but nematode diversity and abundance were reduced or altered in captive-reared animals. Evolutionary and ecological factors are likely responsible for the variation in nematode distributions in the wild. This possible species/island-parasite co-evolution has not been considered previously for Galápagos tortoises. We recommend that conservation efforts, such as the current Galápagos tortoise captive breeding/rearing and release programme, be managed with respect to parasite biogeography and host-parasite co-evolutionary processes in addition to the biogeography of the host.

Fig 1. Map showing the locations where faecal samples were collected on the Galápagos Islands.

Squares depict captive populations, circles depict wild populations. Inset—map of South America highlighting (in square) the location of the Galápagos Islands relative to the mainland.

Fig 2. Photomicrographs of nematode eggs found in Galápagos giant tortoise faecal samples.

(a) Small strongyle egg. (b) Large strongyle egg. (c) Ascarid egg. (d) Oxyurid egg. N.B. No trichurid eggs were found in samples returned to the laboratory for photography.

Fig 3. Relative frequency of nematode egg types according to location.

Relative frequencies are expressed as a percentage.

Fig 4. Photomicrographs of a male Atractis sp. nematode detected in a Galápagos giant tortoise faecal sample.

Scale bars = 50 μm. (a) Anterior end of body showing wider sclerotised anterior region of the oesophagus [or pharynx] (arrows). (b) Lateral view of posterior end of male showing caudal papillae (closed arrows), gubernaculum (arrowhead) and distal ends of spicules (open arrows).

Fig 5. Photomicrographs of a male Labiduris sp. nematode detected in a Galápagos giant tortoise faecal sample.

Scale bars = 50 μm. (a) Anterior end showing the subventral lips (arrow) with a posteriorly-directed fringe along the median edge (arrowhead). (b) Posterior end showing terminal appendix (arrowhead) and ventro-lateral papillae (arrows).